Software Requirements. Descriptions and specifications of a system. Ian Sommerville 2000 Software Engineering, 6th edition.

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1 Software Requirements Descriptions and specifications of a system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 1

2 Objectives To introduce the concepts of user and system To describe functional and non-functional To explain two techniques for describing system To explain how software may be organised in a document Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 2

3 Topics covered Functional and non-functional User System The software document Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 3

4 Requirements engineering The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed The themselves are the descriptions of the system services and constraints that are generated during the engineering process Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 4

5 What is a requirement? It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification This is inevitable as may serve a dual function May be the basis for a bid for a contract - therefore must be open to interpretation May be the basis for the contract itself - therefore must be defined in detail Both these statements may be called Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 5

6 Requirements abstraction (Davis) If a company wishes to let a contract for a large software development project, it must define its needs in a sufficiently abstract way that a solution is not pre-defined. The must be written so that several contractors can bid for the contract, offering, perhaps, different ways of meeting the client organisation s needs. Once a contract has been awarded, the contractor must write a system definition for the client in more detail so that the client understands and can validate what the software will do. Both of these documents may be called the document for the system. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 6

7 Types of requirement User Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers System A structured document setting out detailed descriptions of the system services. Written as a contract between client and contractor Software specification A detailed software description which can serve as a basis for a design or implementation. Written for developers Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 7

8 Definitions and specifications Requirements definition 1. The software must provide a means of representing and 1. accessing external files created by other tools. Requirements specification 1.1 The user should be provided with facilities to define the type of 1.2 external files. 1.2 Each external file type may have an associated tool which may be 1.2 applied to the file. 1.3 Each external file type may be represented as a specific icon on 1.2 the user s display. 1.4 Facilities should be provided for the icon representing an 1.2 external file type to be defined by the user. 1.5 When a user selects an icon representing an external file, the 1.2 effect of that selection is to apply the tool associated with the type of 1.2 the external file to the file represented by the selected icon. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 8

9 Requirements readers User Client managers System end-users Client engineers Contractor managers System architects System System end-users Client engineers System architects Software developers Software design specification Client engineers (perhaps) System architects Software developers Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 9

10 Functional and non-functional Functional Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations. Non-functional constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc. Domain Requirements that come from the application domain of the system and that reflect characteristics of that domain Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 10

11 Functional Describe functionality or system services Depend on the type of software, expected users and the type of system where the software is used Functional user may be high-level statements of what the system should do but functional system should describe the system services in detail Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 11

12 Examples of functional The user shall be able to search either all of the initial set of databases or select a subset from it. The system shall provide appropriate viewers for the user to read documents in the document store. Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account s permanent storage area. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 12

13 Requirements imprecision Problems arise when are not precisely stated Ambiguous may be interpreted in different ways by developers and users Consider the term appropriate viewers User intention - special purpose viewer for each different document type Developer interpretation - Provide a text viewer that shows the contents of the document Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 13

14 Requirements completeness and consistency In principle should be both complete and consistent Complete They should include descriptions of all facilities required Consistent There should be no conflicts or contradictions in the descriptions of the system facilities In practice, it is impossible to produce a complete and consistent document Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 14

15 Non-functional Define system properties and constraints e.g. reliability, response time and storage. Constraints are I/O device capability, system representations, etc. Process may also be specified mandating a particular CASE system, programming language or development method Non-functional may be more critical than functional. If these are not met, the system is useless Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 15

16 Non-functional classifications Product Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. Organisational Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation, etc. External Requirements which arise from factors which are external to the system and its development process e.g. interoperability, legislative, etc. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 16

17 Non-functional requirement types Non-functional Product Organizational External Efficiency Reliability Portability Interoperability Ethical Usability Delivery Implementation Standards Legislative Performance Space Privacy Safety Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 17

18 Non-functional examples Product requirement 4.C.8 It shall be possible for all necessary communication between the APSE and the user to be expressed in the standard Ada character set Organisational requirement The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95 External requirement The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 18

19 Goals and Non-functional may be very difficult to state precisely and imprecise may be difficult to verify. Goal A general intention of the user such as ease of use Verifiable non-functional requirement A statement using some measure that can be objectively tested Goals are helpful to developers as they convey the intentions of the system users Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 19

20 Examples A system goal The system should be easy to use by experienced controllers and should be organised in such a way that user errors are minimised. A verifiable non-functional requirement Experienced controllers shall be able to use all the system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 20

21 Requirements measures Property Speed Size Ease of use Reliability Robustness Portability Measure Processed transactions/second User/Event response time Screen refresh time K Bytes Number of RAM chips Training time Number of help frames Mean time to failure Probability of unavailability Rate of failure occurrence Availability Time to restart after failure Percentage of events causing failure Probability of data corruption on failure Percentage of target dependent statements Number of target systems Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 21

22 Requirements interaction Conflicts between different non-functional are common in complex systems Spacecraft system To minimise weight, the number of separate chips in the system should be minimised To minimise power consumption, lower power chips should be used However, using low power chips may mean that more chips have to be used. Which is the most critical requirement? Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 22

23 Domain Derived from the application domain and describe system characterisics and features that reflect the domain May be new functional, constraints on existing or define specific computations If domain are not satisfied, the system may be unworkable Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 23

24 Library system domain There shall be a standard user interface to all databases which shall be based on the Z39.50 standard. Because of copyright restrictions, some documents must be deleted immediately on arrival. Depending on the user s, these documents will either be printed locally on the system server for manually forwarding to the user or routed to a network printer. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 24

25 Train protection system The deceleration of the train shall be computed as: D train = D control + D gradient where D gradient is 9.81ms 2 * compensated gradient/alpha and where the values of 9.81ms 2 /alpha are known for different types of train. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 25

26 Domain problems Understandability Requirements are expressed in the language of the application domain This is often not understood by software engineers developing the system Implicitness Domain specialists understand the area so well that they do not think of making the domain explicit Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 26

27 User Should describe functional and non-functional so that they are understandable by system users who don t have detailed technical knowledge User are defined using natural language, tables and diagrams Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 27

28 Problems with natural language Lack of clarity Precision is difficult without making the document difficult to read Requirements confusion Functional and non-functional tend to be mixed-up Requirements amalgamation Several different may be expressed together Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 28

29 Database requirement 4.A.5 The database shall support the generation and control of configuration objects; that is, objects which are themselves groupings of other objects in the database. The configuration control facilities shall allow access to the objects in a version group by the use of an incomplete name. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 29

30 Editor grid requirement 2.6 Grid facilities To assist in the positioning of entities on a diagram, the user may turn on a grid in either centimetres or inches, via an option on the control panel. Initially, the grid is off. The grid may be turned on and off at any time during an editing session and can be toggled between inches and centimetres at any time. A grid option will be provided on the reduce-to-fit view but the number of grid lines shown will be reduced to avoid filling the smaller diagram with grid lines. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 30

31 Requirement problems Database includes both conceptual and detailed information Describes the concept of configuration control facilities Includes the detail that objects may be accessed using an incomplete name Grid requirement mixes three different kinds of requirement Conceptual functional requirement (the need for a grid) Non-functional requirement (grid units) Non-functional UI requirement (grid switching) Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 31

32 Structured presentation 2.6 Grid facilities The editor shall provide a grid facility where a matrix of horizontal and vertical lines provide a background to the editor window. This grid shall be a p assive grid where the alignment of entities is the user's responsibility. Rationale: A grid helps the user to create a tidy diagram with well-spaced entities. Although an active grid, where entities 'snap-to' grid lines can be useful, the positioning is imprecise. The user is the best person to decide where entities should be positioned. Specification: ECLIPSE/WS/Tools/DE/FS Section 5.6 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 32

33 Detailed user requirement Adding nodes to a design The editor shall provide a f acility for users to add nodes of a specified type to their design The sequence of actions to add a node should be as follows: 1. The user should select the type of node to be added. 2. The user should move the cursor to the approximate node position in the diagram and indicate that the node symbol should be added at that point. 3. The user should then drag the node symbol to its final position. Rationale: The user is the best person to decide where to position a node on the diagram. This approach gives the user direct control over node type selection and positioning. Specification: ECLIPSE/WS/Tools/DE/FS. Section Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 33

34 Guidelines for writing Invent a standard format and use it for all Use language in a consistent way. Use shall for mandatory, should for desirable Use text highlighting to identify key parts of the requirement Avoid the use of computer jargon Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 34

35 System More detailed specifications of user Serve as a basis for designing the system May be used as part of the system contract System may be expressed using system models discussed in Chapter 7 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 35

36 Requirements and design In principle, should state what the system should do and the design should describe how it does this In practice, and design are inseparable A system architecture may be designed to structure the The system may inter-operate with other systems that generate design The use of a specific design may be a domain requirement Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 36

37 Problems with NL specification Ambiguity The readers and writers of the requirement must interpret the same words in the same way. NL is naturally ambiguous so this is very difficult Over-flexibility The same thing may be said in a number of different ways in the specification Lack of modularisation NL structures are inadequate to structure system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 37

38 Alternatives to NL specification Notation Structured natural language Design description languages Graphical notations Mathematical specifications Description This approach depends on defining standard forms or templates to express the specification. This approach uses a language like a programming language but with more abstract features to specify the by defining an operational model of the system. A graphical language, supplemented by text annotations is used to define the functional for the system. An early example of such a graphical language was SADT (Ross, 1977; Schoman and Ross, 1977). More recently, usecase descriptions (Jacobsen, Christerson et al., 1993) have been used. I discuss these in the following chapter. These are notations based on mathematical concepts such as finite-state machines or sets. These unambiguous specifications reduce the arguments between customer and contractor about system functionality. However, most customers don t understand formal specifications and are reluctant to accept it as a system contract. I discuss formal specification in Chapter 9. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 38

39 Structured language specifications A limited form of natural language may be used to express This removes some of the problems resulting from ambiguity and flexibility and imposes a degree of uniformity on a specification Often bast supported using a forms-based approach Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 39

40 Form-based specifications Definition of the function or entity Description of inputs and where they come from Description of outputs and where they go to Indication of other entities required Pre and post conditions (if appropriate) The side effects (if any) Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 40

41 Form-based node specification ECLIPSE/Workstation/Tools/DE/FS/3.5.1 Function Add node Description Adds a node to an existing design. The user selects the type of node, and its position. When added to the design, the node becomes the current selection. The user chooses the node position by moving the cursor to the area where the node is added. Inputs Node type, Node position, Design identifier. Source Node type and Node position are input by the user, Design identifier from the database. Outputs Design identifier. Destination The design database. The design is committed to the database on completion of the operation. Requires Design graph rooted at input design identifier. Pre-condition The design is open and displayed on the user's screen. Post-condition The design is unchanged apart from the addition of a node of the specified type at the given position. Side-effects None Definition: ECLIPSE/Workstation/Tools/DE/RD/3.5.1 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 41

42 PDL-based definition Requirements may be defined operationally using a language like a programming language but with more flexibility of expression Most appropriate in two situations Where an operation is specified as a sequence of actions and the order is important When hardware and software interfaces have to be specified Disadvantages are The PDL may not be sufficiently expressive to define domain concepts The specification will be taken as a design rather than a specification Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 42

43 Part of an ATM specification class ATM { // declarations here public static void main (String args[]) throws InvalidCard { try { thiscard.read () ; // may throw InvalidCard exception pin = KeyPad.readPin () ; attempts = 1 ; while (!thiscard.pin.equals (pin) & attempts < 4 ) { pin = KeyPad.readPin () ; attempts = attempts + 1 ; } if (!thiscard.pin.equals (pin)) throw new InvalidCard ("Bad PIN"); thisbalance = thiscard.getbalance () ; do { Screen.prompt (" Please select a service ") ; service = Screen.touchKey () ; switch (service) { case Services.withdrawalWithReceipt: receiptrequired = true ; Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 43

44 PDL disadvantages PDL may not be sufficiently expressive to express the system functionality in an understandable way Notation is only understandable to people with programming language knowledge The requirement may be taken as a design specification rather than a model to help understand the system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 44

45 Interface specification Most systems must operate with other systems and the operating interfaces must be specified as part of the Three types of interface may have to be defined Procedural interfaces Data structures that are exchanged Data representations Formal notations are an effective technique for interface specification Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 45

46 PDL interface description interface PrintServer { // defines an abstract printer server // requires: interface Printer, interface PrintDoc // provides: initialize, print, displayprintqueue, cancelprintjob, switchprinter void initialize ( Printer p ) ; void print ( Printer p, PrintDoc d ) ; void displayprintqueue ( Printer p ) ; void cancelprintjob (Printer p, PrintDoc d) ; void switchprinter (Printer p1, Printer p2, PrintDoc d) ; } //PrintServer Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 46

47 The document The document is the official statement of what is required of the system developers Should include both a definition and a specification of It is NOT a design document. As far as possible, it should set of WHAT the system should do rather than HOW it should do it Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 47

48 System customers Specify the and read them to check that they meet their needs. They specify changes to the Managers Use the document to plan a bid for the system and to plan the system development process System engineers Use the to understand what system is to be developed System test engineers System maintenance engineers Use the to develop validation tests for the system Use the to help understand the system and the relationships between its parts Users of a document

49 Requirements document Specify external system behaviour Specify implementation constraints Easy to change Serve as reference tool for maintenance Record forethought about the life cycle of the system i.e. predict changes Characterise responses to unexpected events Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 49

50 IEEE standard Introduction General description Specific Appendices Index This is a generic structure that must be instantiated for specific systems Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 50

51 Requirements document structure Introduction Glossary User definition System architecture System specification System models System evolution Appendices Index Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 51

52 Key points Requirements set out what the system should do and define constraints on its operation and implementation Functional set out services the system should provide Non-functional constrain the system being developed or the development process User are high-level statements of what the system should do Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 52

53 Key points User should be written in natural language, tables and diagrams System are intended to communicate the functions that the system should provide System may be written in structured natural language, a PDL or in a formal language A software document is an agreed statement of the system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 53